minor ux tweaks

[maxenglander]

• Unicode display width support: Use Unicode::DisplayWidth for proper handling of wide characters and emoji, fixing layout issues with non-ASCII text
• New vspace() DSL method: Insert vertical spacing (blank lines) between slide elements for better visual control
• Enhanced footer configuration: Add footer.text and footer.show_slide_numbers config options with flexible footer rendering combining custom text and optional slide numbers
• Improved spacing controls: Add pad.entry config option and a new Space entry class for finer-grained control over vertical padding between slide elements
• Better boolean config handling: Fix config validation to properly handle boolean values by checking for TrueClass/FalseClass rather than string class comparison